1. Field of the Invention
The present invention is generally related to optical communication systems, and more particularly to an optical communication system utilizing phase modulation that provides substantially linear recovery of an analog communication signal.
2. Description of the Prior Art
Analog optical communication links are known in the prior art. Conventional optical analog links employ intensity modulation techniques to convey the analog information on an optical beam of light. Such analog optical links are utilized by the cable television industry to transmit video images using the conventional RF analog modulation format for television video. Intensity detection at the receiver using conventional photodetectors enables the light intensity to be linearly converted to an analog voltage corresponding to the signal that is to be transmitted by the link. However, inherent to these analog intensity modulation optical links is an acceptance of a non linearity associated with the intensity modulators used in the transmitter. Mach-Zehnder intensity modulators, which are commonly employed in optical intensity modulation analog links, have a non linear transfer function that yields a sinusoidal intensity variation with a linearly changing applied analog modulation voltage. Similarly, electro absorption modulators also yield a non linear intensity variation to a linearly applied analog modulation voltage.
This inherent non linearity associated with intensity modulators has led to a consideration of using optical phase modulation in the transmitter as an alternative to intensity modulation. Optical phase modulators that can achieve a linear change in the state of the optical phase with a linearly changing analog modulation voltage are known in the art. Modulators can be made from electro optic materials that change their refractive index linearly with applied electric field supplied by a linearly changing analog modulation voltage. The linearly changing refractive index causes the optical path length through the modulator to linearly change. This linearly changing optical path length causes a linearly changing state of optical phase corresponding with a linearly changing analog modulation voltage. Thus, an optical phase modulator can be used in the transmitter to deliver a linearly varying optical signal in contrast to the inherent non linearity associated with intensity modulators.
The utilization of a linear phase modulator in an analog optical communication link requires that the state of optical phase be detected at the receiver. Conventional approaches for this utilize optical interference techniques that cause the phase varying light to become detectable with photodetectors as intensity variations. A common approach used for optical phase state detection is to interfere the phase modulated communication light with an unmodulated reference beam of light that has been split from the initial light source prior to applying the phase modulation. The process of utilizing optical interference techniques to detect the state of optical phase leads to a non linear sinusoidal intensity variation that corresponds to the linearly varying state of optical phase. Thus, this conventional phase detection process leads to a non linearity in the detected analog signal. This non linearity inherent in the conventional phase detection process negates the linearity achieved by the phase modulator and results in an analog optical communication link that is as non linear as the conventional intensity modulation analog optical link. Thus, all analog optical communication links are degraded in performance by an inherent non linearity that distorts the original analog signal that is to be conveyed.
What is needed, therefore, is an analog optical communications system that is capable of detecting the state of optical phase of a phase modulated communication signal in a way so as to produce an analog voltage signal that is linearly related to the state of optical phase of the phase modulated optical signal. Such an analog optical communications system thus will be capable of conveying an analog signal without any non linear distortion.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally an optical communication system for communicating an analog communication signal.
Briefly, the optical communication system comprises an optical transmitter and an optical receiver separated by an analog optical link. The transmitter comprises means for generating a reference light beam, means for generating a shifted light beam that is shifted in frequency from the frequency of said reference light beam by a predetermined radio frequency (RF) and means responsive to the analog communication signal to generate a phase modulated communication beam.
The optical receiver comprises first means responsive to the reference light beam and the shifted light beam to interferometrically combine these two beams to provide a first heterodyne signal, second means responsive to the first heterodyne signal to produce a first electrical signal at the optical beat frequency corresponding to the RF, third means responsive to the communication light beam and the shifted light beam to interferometrically combine the communication light beam with the shifted light beam to provide a second heterodyne signal including information regarding the state of phase of the communication light beam, fourth means responsive to the second heterodyne signal to produce a second electrical signal at the optical beat frequency corresponding to the RF with a phase corresponding to the state of phase of the communication light beam, and fifth means responsive to the first and second heterodyne signals to provide an output signal that corresponds to the state of optical phase of the communication light beam. More particularly, the detection of the relative phase of the RF signals is performed by a digital dividing technique that provides a high resolution determination over several optical wavelengths. Both the RF beat frequency signal and the RF modulation signal are converted to digital waveforms that are digitally divided, preferably by an equal number of powers of two, so that the resulting outputs are square waves. The divided signals are applied to an exclusive OR gate that provides a pulse waveform having a duty cycle that reflects the relative phase of the two RF frequencies. The pulse waveform is filtered to create an average voltage that linearly corresponds to the particular state of phase in the communication light beam and the analog communication signal.
Other aspects of the invention separately describe the optical transmitter and the optical receiver.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description.